Fuel cells are used below as a particularly favorable example of areas of application of the invention. It will be understood, however, that the invention also finds advantageous application for supplying gas to other gas consumers such as internal combustion engines.
Recently, fuel cells have attracted greater attention and become increasingly valued as an energy source in a number of different applications. In recent years, for example, various vehicles such as buses and private cars have been developed, which are driven entirely or partly by means of fuel cells. However, fuel cell technology still has certain problems as far as efficiency and economy are concerned. Therefore, a great deal of research and development work is being carried out at present in order to develop the technology further and to improve and render more effective the various subsystems included in a fuel cell system. One important such subsystem consists of the devices which are used for supplying compressed air or other gas to the fuel cell. For good functioning and effectiveness of the fuel cell, it is of great importance that the air is supplied to the fuel cell with a constant pressure and flow. Moreover, as for all component subsystems, it is of utmost importance that the air-supplying devices operate with high efficiency because the efficiency of each subsystem directly influences the overall efficiency of the whole fuel cell system.
The double-screw compressor has proved to be very well suited for being used for supplying compressed air to fuel cells because it has a good capacity for generating a uniform air flow under constant pressure. The double-screw compressor comprises two parallel interacting rotors in the form of a male rotor and a female rotor which, in engagement with one another, press the air under gradually increased pressure from the inlet of the compressor to its outlet. The rotors can be designed and driven so that they rotate at the same speed or at multiples of the speed of one another. In order to obtain good efficiency by avoiding leakage of air in the direction toward the inlet, it is of great importance that the backlash between the two rotors and between each rotor and the surrounding compressor housing is as small as possible. At the same time, all contact between the rotors must be avoided as such contact leads to the rotors being damaged or to the compressor as a whole breaking down.
In order to maintain the correct backlash, which is as small as possible, between the rotors, the synchronization of the rotational speed of the rotors is therefore of utmost importance. This synchronization is usually brought about by means of a toothed gearing which comprises two interacting gearwheels which are fixed on the shaft of the respective rotor. The ratio of the gearing is of course selected so that it corresponds to the intended ratio between the rotational speed of the two rotors. The gearwheels are usually designed with conventional inclined involute teeth and have the standardized nominal pressure angle of 20°. The toothed gearing also comprises a toothed gearing housing with opposite end walls in which the gearwheel shafts are mounted. For reasons of cost and manufacturing techniques, it is desirable to design the toothed gearing housing with its end walls made of aluminum while, for reasons of strength, the gearwheels are preferably made of steel.
The known double-screw compressor described above with a conventional toothed gearing has a number of advantages compared with other compressors and pumps as far as supplying air to fuel cells is concerned. The exacting requirements of the fuel cell application in terms of efficiency and precision nevertheless result in certain problems. These problems are also associated with the great temperature ranges within which fuel cell systems and the subsystems included in them, such as the screw compressor, have to be capable of operating. This temperature range is great especially when the fuel cell equipment is used as a drive source for vehicles because the equipment then has to be capable of functioning well down to ambient temperatures as low as −50° C. and up to ambient temperatures of around +50° C. and also beyond this to operating temperatures which can be as much as +200° C. on account of the self-heating of the equipment.
In order to maintain the well-defined small backlash between the male screw and female screw during operation of the double-screw compressor, it is of great importance that the backlash between the interacting teeth in the toothed gearing is on the one hand kept as small as possible and on the other hand kept as constant as possible. In the previously known double-screw compressors where the involute teeth of the toothed gearing have the usual nominal pressure angle of 20°, however, the backlash will vary greatly when the temperature of the component parts varies within the range indicated above. Owing to the fact that the end walls and gearwheels of the toothed gearing are made from materials with different thermal expansion coefficients, these parts will be deformed to different degrees when the temperature varies. The end walls made of aluminum will expand more than the gearwheels made of steel when the temperature rises. In this way, the center distance between the gearwheel shafts mounted in the end walls will increase more than the combined pitch or reference radii of the two gearwheels will when the temperature rises. As a result, the backlash between the gearwheels increases when the temperature rises and in a corresponding way decreases when the temperature falls. This phenomenon constitutes a serious problem because increased backlash gives rise to impaired synchronization of the rotors, which leads to increased gas leakage and impaired efficiency of the double-screw compressor and may moreover lead to the rotors coming into direct contact with one another, the risk of breakdown then being great. On the other hand, reduced backlash can lead to wear of the teeth and, if the backlash becomes negative, to jamming between the teeth with a risk of breakdown. In particular when the fuel cell equipment is used in vehicles, it is precisely problems at low temperatures which are especially serious, because the double-screw compressor should be designed for normal operation at operating temperatures of around +100° C. and moreover so as to handle cold starts at ambient temperatures as low as −50° C.
DE 44 07 696 also describes a previously known cylindrical toothed gearing for vehicle transmissions, in which the gearwheel geometry can be adapted, for example by small corrections of a pressure angle of the order of 26° of the gearwheel pair, in order to shift the optimum operating temperature range of the gearing from ambient temperature to the temperature range in which the gearing normally operates. The object is stated to be to reduce the risk of damage occurring on the tooth flanks.